The Hedgehog signaling pathway plays a key role during embryonic development and, post-embryonically, in regulating adult stem cell renewal and regeneration. Aberrant activation of the Hedgehog pathway has been linked to the development and progression of several human malignancies, including leukemia, lung, pancreatic, prostate, and colon cancers. Chemical modulators of the Hedgehog pathway have proven essential for elucidating its function as well as identifying promising clinical candidates for the treatment of Hedgehog pathway-related cancers. The majority of currently available Hedgehog pathway antagonists, however, target the transmembrane receptor Smoothened (Smo) and, to a minor extent, downstream pathway components involved in the activation of Gli transcription factors and transcription of Gli-regulated genes. In contrast, a fundamental gap remains with respect to chemical agents that can potently interfere with upstream events of Hedgehog pathway activation, namely the interaction between the Hedgehog signaling proteins and the Patched receptor, which is responsible for de-repression of Smo and consequent induction of Gli-regulated genes. The goal of this project is to develop a new class of macrocyclic organo-peptide inhibitors of the Hedgehog/Patched protein-protein interaction. To achieve this goal, we will utilize a modular and efficient strategy for creating vast and highly diverse libraries of functionally complex macrocycles in combination with a powerful, high-throughput system for functional screening of these libraries. The Hedgehog-targeting macrocycles isolated in this manner will be evaluated in secondary in vitro and cell-based assays in order to characterize their inhibitory potency, selectivity, and ability to block Hedgeho pathway signaling in mammalian cells. Ultimately, this research is expected to provide highly needed chemical probes for investigating the Hedgehog signaling pathway in stem cell biology and cancer biology, as well as new lead structures, readily amenable to further optimization, for the development of next-generation anticancer agents. Another relevant contribution of this project will be the implementation of a general, integrated platform for evolving macrocyclic inhibitors of protein complexes, which could be readily applied to a variety of other cancer-related target protein-protein interactions.